Stereospecific reactions Solvents

This stereospecific reaction is, therefore, a rare example of stereochemical control by nucleophilic solvent assistance of an ionization process. 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

It has already been pointed out that lithium and lithium alkyls in non-polar solvents produce a polyisoprene of greater than 90% cis-1,4 content, a very highly stereospecific reaction. In this section a number of systems will be discussed which have been stereospecific in the sense of producing an excess of d or l configurations in the polymer chain backbone. 

The geometry of the transition state is ideal for decomposition and leads to formation of rearranged chloride in a highly stereospecific reaction. Rate data on this system is scant owing to the instability of all but the parent allyl chlorosulfite. The rate of the rearrangement increases rapidly with increased solvent ionizing power . ... 

Use UV inhibitor in polyvinyl chloride, chemosteri-lant for insects, promoting stereospecific reactions, specialty solvent. 

MAJOR USES Used in the production of aramid polyamide fibers, kevlar fabrics and functional fluids UV inhibitor chemosterilant for insects solvent polymerization catalyst and stabilizer flame retardant promotes stereospecific reactions. 

One of the interesting fields of kinetics, opened up by the availability of accurate optical rotatory dispersion and circular ichoism recor ng spectrophotometers, is that of stereospecific reactions in optically-active organic solvents. Both Bosnich et and Haines and... 

The closely related N- arylazoaziridine system (278) decomposes in refluxing benzene to give aryl azides and alkenes, again stereospecifically (70T3245). However, biaryls, arenes and other products typical of homolytic processes are also formed in a competing reaction, although this pathway can be suppressed by the use of a polar solvent and electron withdrawing aryl substituents. 

Chirazymes. These are commercially available enzymes e.g. lipases, esterases, that can be used for the preparation of a variety of optically active carboxylic acids, alcohols and amines. They can cause regio and stereospecific hydrolysis and do not require cofactors. Some can be used also for esterification or transesterification in neat organic solvents. The proteases, amidases and oxidases are obtained from bacteria or fungi, whereas esterases are from pig liver and thermophilic bacteria. For preparative work the enzymes are covalently bound to a carrier and do not therefore contaminate the reaction products. Chirazymes are available form Roche Molecular Biochemicals and are used without further purification. 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

We have investigated the bromo-addition of alkenes and their related compounds with BTMA Br3. Thus, we found that the reaction of alkenes with BTMA Br3 in aprotic solvents such as dichloromethane and chloroform gave 1,2-dibromo adducts in a manner of stereospecific anti-addition, and, in such protic solvents as methanol and acetic acid, gave the corresponding dibromo adducts along with considerable amounts of solvent-incorporated products in regioselective manner (Fig. 18) (ref. 29). 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

---